1. Field of the Invention
The present invention relates to network systems, and particularly to public network systems, such as the Internet. More particularly, the invention relates to methods which improve the caching of streaming multimedia data (e.g., audio and video data) from a content provider over a network to a client's computer.
2. Description of the Related Art
Computer networks such as the Internet are increasingly being used to transmit multimedia data (e.g., audio and video data). In the network-based context, one simple model of producing the information involves the client requesting the downloading of the multimedia data. Once downloaded, the client may then consume, or present, the information. This model is relatively easy to implement, however, it is non-optimal in that the client is required to wait for the downloading to complete before the presentation can begin. This delay can be considerable.
A more sophisticated model of producing information involves a content server at one network site streaming the multimedia information over the network to a client at another site. The client begins to present the information as it arrives (i.e., just-in-time rendering), rather than waiting for the entire data set to arrive before beginning the presentation. At the client computer, the received data is buffered into a cache memory and continuously processed as soon as, or soon after, being received by the client. The advantage of streaming is that the client computer does not have to wait until all the data is downloaded from the server before some of the data is processed and the multimedia output is created.
An example of multimedia data streaming is found in the Real player that is available over the Internet at Universal Resource Locator (“URL”) http://www.real.com The Real audioplayer continuously sends audio data over the Internet from a server computer to the client computers. The audio data is buffered and processed by the client computers while the data is being sent. The client computers process the data by creating an audio output from the audio data.
Applications, such as the Real audioplayer, have conditioned computer network users to expect instantaneous streaming data on demand. The Internet, however, is often unable to deliver streaming data. This inability is most pronounced for video data. The inability to deliver streaming data on demand is due in part to the fact that live and on-demand streaming multimedia (SM) objects are generally delivered over the Internet and other data networks via unicast connections. This architecture has many shortcomings, both from the content provider's point of view and the user or recipient's point of view. From the content provider's point of view, he is faced with a server load that increases linearly with the number of clients. That is, each additional client requesting streaming multimedia data imposes an additional burden upon the content provider to meet the increased demand. From the Internet Service Provider's (ISP's) point of view, streaming multimedia under a unicast architecture poses network congestion problems. From the client's point of view, there is often long delays between the time the video content is requested by a client and the time when the video content actually begins playing (i.e., high start-up latency). In addition to the high start-up latency there also exists unpredictable playback quality due to network congestion.
Web caching has been extensively implemented on the Internet to reduce network load (i.e., bandwidth consumption), server load, and high start-up latency. The utilization of Web caching on the Internet has been extensively studied. For a more detailed discussion of Web caching, see T, Bernesrs-Lee, A. Lutonen, and H. F. Nielsen Meyr, Cern httpd: http://www.w3.org/Daemon/Status.html, 1996; and C. M. Bowman, et al. Harvest: “A scalable, customizable discovery and access system” Technical Report CU-CS-732-94, Dept. of Computer Science, University of Colorado, Boulder, USA, 1994 both references are incorporated by reference herein. See also, D. Wessels, “ICP and the squid cache”, National Laboratory for Applied Network Research, 1999, http://ircache.nlanr.net/Squid; this reference is also incorporated by reference herein. However, current caching systems, like those described above, are restricted to support static web objects such as HTML documents or images. Static web objects are typically small and as such are always cached in their entirety. Current caching methods, therefore, do not adequately support streaming multimedia data (i.e., web objects) such as video and audio SM objects. Streaming multimedia data like video objects, for example, are usually too large to be cached in their entirety. A single, two hour long MPEG movie, for example, requires about 1.4 Gbytes of disk space. Given a fixed investment in disk space, only a few streams could be stored at a cache, thus, decreasing the hit probability and the efficiency of the caching system. A natural solution would be to break video objects into smaller pieces for the purpose of caching. This solution is deficient, however, in that existing caching systems will treat different chunks from the same video object independently, while it might be desirable to consider the logical relationship among the various pieces.
SM objects can be generally differentiated from static web objects in that SM objects consist of multimedia data whose transmission has temporal characteristics such that the transmission rate is explicitly regulated or else the data becomes useless. In addition, the size of SM objects is typically at least an order of magnitude or two larger than that of a static web object, and therefore, do not lend themselves to be cached in their entirety. Given that caches have finite disk space, it is not feasible to statically store more than a few complete SM objects. If there are several simultaneous requests for different SM objects, it is easy to show that the cache will be busy replacing one SM object with another resulting in significant performance degradation.
Based on the foregoing, there is a need for a system and method for enhancing current caching systems to support streaming multimedia over a public network system, such as the Internet.